1. Field of the Invention
This invention generally relates to high-strength, chemically-resistant laminar films; and specifically such films used in pump and valve diaphragms.
2. Background Information
Diaphragm pumps are used in pumping a wide variety of materials especially when the materials are abrasive, have high viscosity, consist of slurries that might damage other pump designs, have a high purity requirement or are not robust. These pumps are often air driven which is advantageous in pumping flammable liquids or in environments where electrically driven equipment could otherwise be hazardous. However, electrically or otherwise mechanically driven designs also find wide utility. Others are driven by incompressible fluids.
Due to the wide range of different materials these pumps are used to move, a correspondingly wide variety of materials are used in the pump construction. These include plastics, metals and composites. For the same reason the critical driving member of such pumps, the diaphragm, typically must be manufactured from a variety of materials.
Chemically resistant layers, such as those made of polytetrafluoroethylene (PTFE), are widely used in industry to protect sensitive parts of machinery or equipment from the corrosive effects of acids or other chemicals. One such use is in pump diaphragms commonly used with air or electrically driven diaphragm pumps. In such diaphragms, an outer PTFE overlay diaphragm is commonly used to protect an inner rubber diaphragm from materials that would cause rapid failure of the rubber part alone. In other cases, the PTFE provides the sole material of construction of the diaphragm.
Polyimide is very useful material for industrial use for machineries and parts because of it's mechanical strength, electrical property and chemical property. However, the tensile strength and endurance against the cycle-bending fatigue for polyimide membrane are easily reduced under certain conditions such as under strong acidic or strong basic media. Thus, a polyimide film alone cannot be used for the diaphragm application for feed pump using for microelectronics chemical such as photo resists and TARC. On the other hand, fluorinated resin including PTFE, FEP, or PFA have extremely high resistivity against any chemicals. However, the materials' respective low mechanical strengths are not satisfactory for the use.
Recently, there are several patents application regarding polyimide and fluorinated resin laminated composite films. One includes PCT Publication No. WO93/1493. It discloses a laminar film structure comprising a polyimide and a fluoropolymer. Such prior art discloses uses for such laminates such as endless belt, motor coil and cable. One of the methods reported in such prior art includes the dispersion of fluorinated resin in organic solvent that was then coated on both sides of polyimide film. The second method is heat-press-bonding method with polyimide and fluorinated resin powder. For either method, the obtained laminated films did not have enough peeling strength between two layers for pump diaphragm applications. Other methods reported included improving peeling strength between two layers by adding titanium complex into the fluorinated layer. This method is not also applicable for microelectronics use because the risk of contamination of metal from the titanium additive.
The preparation methods for laminated film describe above were not satisfied the requirements need for the microelectronics chemical feed pump application, which are high accuracy, cycle bending fatigue endurance, and not producing any contamination such as metal or particle from film fibrillation.
Blends of the fluoropolymer and the dissimilar layer themselves are in some cases employed as an intermediate layer to help bond the two layers together. European Patent Application 0523644 (Kawashima et al.) discloses a plastic laminate having a polyamide resin surface layer and a fluororesin surface layer. The reference recognizes the difficulties encountered when making laminates having a polyamide layer and a fluororesin layer because of the incompatibility of the two materials. The laminate of the reference is prepared by use of an intermediate layer composed of a blend of an aliphatic polyamide resin with a fluorine-containing graft copolymer.
U.S. Pat. No. 6,197,393 discloses a method of bonding substantially non-fluorinated polymeric material to a fluoropolymer utilizing a bonding composition comprising primary or secondary di- or poly-amine and a melt-processable, substantially non-fluorinated component.
It would be desirable to provide a laminar film that has both the mechanical strength and chemical resistant properties sufficient for use in valve and pump diaphragms.